The present invention relates to articles made from elastomer-based material, defined in a general way in the remainder of the present description as elastomeric material or rubber compound; it relates specifically to the rubberized fabrics used for making the said articles and the corresponding reinforcing elements, and is more particularly concerned with pneumatic tires for vehicle wheels.
Rubberized fabrics are a semi-finished product of the greatest importance for the manufacture of products from elastomeric material: in general, they comprise a plurality of filamentary reinforcing elements having at least tensile strength, all being disposed parallel and adjacent to each other, and completely embedded in a sheet of rubber compound.
The said fabric is produced in the form of a continuous piece whose length is indefinite, in other words is far greater than its width, by incorporating in a layer of rubber compound, by known means and processes, the said reinforcing elements orientated in the longitudinal direction of the piece.
The piece is then cut transversely into portions of various sizes, with an angle of cut predetermined as necessary, and these portions are joined side by side to produce a continuous strip of fabric with reinforcing elements orientated at an angle of predetermined value with respect to the longitudinal direction of the strip.
Portions of appropriate size of the said strip form the textile elements used in the making of the product, assembled with other constituent elements of the product: for example, in the case of a pneumatic tire, as will be seen subsequently, these textile elements form the casing ply, the belts, rims, loops and other reinforcing fabrics disposed in various ways in the structure of the pneumatic tire.
The use of rubberized fabric enables the reinforcing elements to be disposed within the structure of the product, by the desired method and in the desired position, in a precise, simple and economical way.
The said filamentary reinforcing elements may be made from many different materials, textile or metal, natural or synthetic, and may consist either of monofilaments, in other words single wires or strands, or bundles of monofilaments placed together, or cords comprising a plurality of strands or bundles of monofilaments wound spirally with respect to each other in predetermined configurations.
In the present description, the word xe2x80x9cwirexe2x80x9d denotes the normal metal wire used in pneumatic tire technology, while the word xe2x80x9cmonofilamentxe2x80x9d or xe2x80x9cflossxe2x80x9d denotes textile fibres, natural or synthetic, which are also commonly used in the technology of rubber products.
Articles incorporating these reinforcing fabrics are, for example, pneumatic tires, transmission belts, conveyor belts and hoses for carrying fluids which may be pressurized.
The performance of rubber articles, in terms of service life, mechanical strength and, in the case of pneumatic tires, road performance, depends principally on the mechanical characteristics of the reinforcing elements which have the function of withstanding the stresses put upon it, which, in the case of a pneumatic tire, include the inflation pressure and the stresses arising from use on the road.
The pneumatic tire to which the invention relates comprises, in a general way, a casing of toroidal shape substantially consisting of at least one rubberized fabric (casing ply) having its edges wrapped around a pair of annular metal cores which are circumferentially inextensible (the ordinary bead cores), a tread band disposed on the crown of the said casing and a breaker structure interposed between the casing and the tread, formed by one or more radially superimposed layers of rubberized fabric (belts), wherein the said rubberized fabrics comprise a plurality of reinforcing elements, in other words the conventional cords, which have at least tensile strength, are parallel and adjacent to each other and orientated in a predetermined direction, and are incorporated in a sheet of rubber compound.
More precisely, the cords of the casing ply preferably lie in radial planes including the axis of rotation of the tire, while the cords of the belts preferably lie at angles to each other in adjacent belts and may be orientated parallel to the equatorial plane of the tire in the radially outermost belt.
In tires for motor vehicles, the said cords are preferably of the textile type in the casing ply and in the radially outermost belt, while they are metal cords in the radially innermost belts.
The textile cords are identified by a numerical symbol which represents the count of the fibre used and the number of strands used to form the cord. The term xe2x80x9cstrandxe2x80x9d indicates a bundle of flosses or monofilaments twisted together; the term xe2x80x9ccountxe2x80x9d indicates the weight in grams of a length of 10,000 meters of fibre, expressed in dTex units, or the weight in grams per length of 9000 meters, expressed in deniers.
At the present time, the textile cords used in rubberized fabrics for pneumatic tires comprise two or three strands twisted together and have a count in excess of 900 dTex units: for example, rayon cords with a count of 1840/2, 1840/3 or 2440/2, nylon6 or nylon66 cords with a count of 940/2 and 1400/2, or synthetic fibre (aramid or polyester) cords with a count of 1670/2, 1100/2 or 2440/2 are used for casing fabrics.
Known cords usually have diameters of the order of 0.7 mm and are incorporated in fabrics which have fabric thicknesses of approximately 1 mm, with a density of cords in the fabric of between 60 and 120 cords/dm, in other words cords per decimeter.
Fabrics for pneumatic tires reinforced with the said cords and corresponding tires are described, for example, in U.S. Pat. Nos. 3,616,832 and 3,929,180.
More particularly, U.S. Pat. No. 3,616,832 describes a tire with a radial casing formed by two casing plies provided with rayon cords with a count of 1650/2 denier and a density of 38 cords per 5 cm, in other words 76 cords/dm.
The belt assembly is formed by four fabric belts with rayon cords with a count of 1650/3 denier and 30 cords per 5 cm, in other words 60 cords/dm.
The tire is compared with another tire comprising the same rayon casing and a belt assembly formed by four fabric belts provided with cords of polyethylene naphthalene 2,6-dicarboxylate, known as PEN, with a count of 2000/2 denier, distributed with a density of 30 cords per 5 cm, in other words 60 cords/dm.
The cited patent describes, as an alternative to the rayon casing and with the same breaker structure with PEN cords, further casings with the same number of plies, comprising in one case polyethylene terephthalate, known as PET, fibre cords with a count of 1000/3 denier and a density of 35 cords per 5 cm, in other words 70 cords/dm, and in a second case having nylon6 cords with a count of 1260/2 denier and a density of 33 cords per 5 cm, in other words 66 cords/dm.
U.S. Pat. No. 3,929,180 describes, with a series of comparative examples, two pneumatic tires, both having a radial casing formed by two plies reinforced with 35 cords per 5 cm; the casing of the first tire comprises rayon cords having a count of 1650/3 denier, and that of the second has PEN cords with a count of 2000/2 denier. In both tires, the belt assembly is formed by four fabric belts with PEN cords having a count of 1000/2 denier, distributed with a density of 35 cords per 5 cm.
Given all this, the present development of pneumatic tire technology towards increasingly specialized products capable of providing increasingly high performance is a powerful stimulus to the search for increasingly lightweight tires.
In this context, the applicant became aware that one contribution to the solution of the problem might lie in the reduction of the weight of the pneumatic tire casing, though naturally without negative effects on the characteristics of mechanical strength and of the behaviour of the tire in use, and particularly on directional stability and comfort.
In this regard, it should be noted that the problem faced here is neither simple nor obvious, since the present tendency in pneumatic tire technology is to maintain the present weights of the reinforcing fabrics instead of decreasing them, particularly in respect of fabrics reinforced with textile cords and even more particularly in respect of the casing plies.
The reason for this is that in present-day rubberized fabrics, the percentage by volume of the reinforcing elements is at least 45% of the total volume of the fabric, and this percentage cannot apparently be reduced without incurring the risk of compromising the mechanical strength of the tire and/or its quality.
Where the reinforcing cords are concerned, at the present time these have the counts cited above, corresponding to diameters of not less than 0.7 mm. The use of lower counts is not recommended, since a return to the aforesaid volume of strong material would require densities such that adequate rubberizing of the cords, particularly between the axially adjacent cords, would be impossible.
On the other hand, it is also considered to be impossible to reduce the quantity of rubber in the fabric, which is already so thin that it cannot be reduced further without leaving the reinforcing cords at least partially uncovered by the rubber, with all the consequent disadvantages in terms of quality and risks in terms of safety for the finished product.
Given the present state of the art, the applicant has unexpectedly found that, by reducing both the count and the percentage by volume of the said reinforcing elements having at least tensile strength with respect to the total volume of the fabric, it is possible to increase or at least maintain the performance of the said articles, while obtaining in each case an advantageous reduction of the weight of the articles made from elastomeric material comprising this rubberized fabric.
This characteristic of the invention is unexpected, in that an improvement of mechanical strength and performance associated with a decrease in the percentage by volume of the strong part of the fabric, consisting solely of the filamentary reinforcing elements incorporated in the elastomeric matrix, was not predictable, the rubber compound being commonly considered to be lacking in tensile strength.
The result obtained is therefore unexpected: after the detailed description of embodiments according to the invention, a possible technical explanation of the improvements found will be given.
For the purposes of the present invention, the percentage by volume of the reinforcing elements is found, with reference to the cross section of a fabric layer of unit width, by relating the volume of the reinforcing elements (the density of the elements multiplied by xcfx80/4 of the square of their diameter) to the total volume of the fabric (the thickness of the rubber sheet covering the reinforcing elements on both surfaces of the fabric added to the diameter of the reinforcing element).
Again, for the purposes of the present invention, the significant volume of the fabric, with respect to which the critical values identified and described below are calculated, with reference to the finished product, is that of a fabric which has a total thickness of 0.8 mm, with an identical thickness of compound on both surfaces of the layer of adjacent cords.
In a first aspect, the invention therefore relates to a rubberized reinforcing fabric for articles made from elastomeric material and similar, comprising a plurality of filamentary elements having at least tensile strength, parallel and adjacent to each other and incorporated in an elastomeric material, characterized in that the said filamentary elements constitute a maximum of 40%, and more preferably not more than 35%, of the volume of the said fabric.
Preferably the said filamentary reinforcing elements are textile cords with a count of between 420/2 and 840/2 dtex; the said cords are incorporated in the rubberized fabric with a density of not less than 125 cords/dm, in other words 125 cords per decimeter of width of the fabric, and more preferably between 125 and 280 cords/dm, in such a way as to provide an adequate tensile strength. Preferably the fabric incorporating the said cords has a total thickness of less than 0.8 mm.
In a preferred embodiment the rubberized fabric comprises a plurality of textile cords parallel and adjacent to each other, incorporated in the elastomeric material of the fabric, and is characterized in that the said cords have a count of 550/2 dTex.
Preferably they are distributed in the fabric with a density of between 128 and 224 cords/dm.
In a different aspect, the invention relates to various articles made from elastomeric material which comprise this reinforcing fabric, for example transmission belts, hoses for fluids and conveyor belts, and in particular pneumatic tires, with which the invention is particularly concerned.
According to a third aspect, the invention relates to a pneumatic tyre for a vehicle wheel, reinforced with rubberized fabric strips, which comprises a casing of toroidal shape substantially consisting of at least one casing ply formed by a rubberized fabric whose axially opposite edges are wrapped around a pair of bead cores, for anchoring the tyre to a corresponding mounting rim, a tread band disposed on the crown of the said casing and a breaker structure interposed between the casing and the tread band and formed by one or more radially superimposed belts of rubberized fabric, wherein at least one of the said strips of rubberized fabric comprises a plurality of textile reinforcing cords which have at least tensile strength, are parallel and adjacent to each other and oriented in a predetermined direction, are incorporated in a sheet of rubber compound, and have a count between 420/2 dTex and 840/2 dTex.
In a preferred embodiment, the count of the said cords is 550/2 dtex.
Preferably the said textile cords are distributed in the said fabric with a density of not less than 125 cords/dm. Advantageously, the said textile cords are made from PEN or PET material or aramid fibres.
In a preferred embodiment of the tire, the said rubberized fabric constitutes the casing ply and comprises the said cords disposed preferably in radial planes including the axis of rotation of the tire.
In a second preferred embodiment of the tire, the breaker structure comprises at least one belt of rubberized fabric reinforced with the said cords, the belt being preferably disposed in the radially outermost position, with the cords more preferably orientated in the circumferential direction.
According to alternative forms of preferred embodiments, the casing of the said tire comprises other strips of the said rubberized fabric, for example folded in a loop around at least one bead core or as a reinforcing tape in a position axially outside the said bead cores.